In recent years, with the spread of portable electronic devices such as portable telephones, notebook computers and the like, there has been a strong demand for the development of compact, lightweight non-aqueous electrolyte rechargeable batteries that have a high energy density. Moreover, there is a strong demand for the development of high-output rechargeable batteries as the electrical power source for electric automobiles such as hybrid electric automobiles, plug-in hybrid electrical automobiles, and battery-powered electric automobiles.
As rechargeable batteries that can meet such demands, there are lithium-ion rechargeable batteries as one kind of a non-aqueous electrolyte rechargeable battery. A lithium-ion rechargeable battery has an anode, a cathode, an electrolyte and the like, and as an active material used in the anode and cathode, a material in which lithium can be absorbed or desorbed is used.
Among lithium-ion rechargeable batteries, lithium-ion batteries that use a layered or spinel type lithium transition metal composite oxide for the cathode material are able to obtain a 4V class voltage, so currently research and development of such batteries is actively being performed as batteries having a high energy density, and some have been put into practical use.
As the cathode material for such lithium-ion batteries, lithium cobalt composite oxide (LiCoO2) for which the composition is comparatively simple, lithium nickel composite oxide (LiNiO2) that uses nickel that is less expensive than cobalt, lithium nickel cobalt manganese composite oxide (LiNi1/3Co1/3Mn1/3O2), lithium manganese composite oxide (LiMn2O4) that uses manganese, lithium nickel manganese composite oxide (LiNi0.5Mn0.5O2) and the like have been proposed.
In order to obtain a lithium-ion rechargeable battery having excellent cycling characteristics and output characteristics, it is necessary for the cathode active material to be constructed by particles having a small particle size and narrow particle size distribution. That is because, particles having a small particle size have a large specific surface area, and when used as a cathode active material, not only is it possible to sufficiently maintain the reaction surface area for reacting with the electrolyte, but it is also possible to make a thin cathode, and to shorten the migration length of lithium ions between the cathode and anode, so it is possible to reduce the cathode resistance. Moreover, for particles having a narrow particle size distribution, the voltage that is applied to the particles inside the electrode can be uniform, so it is possible to suppress a decrease in the battery capacity due to selective degradation of fine particles.
In order to further improve the output characteristics, making the structure of the cathode active material a hollow structure is effective. With this kind of cathode active material, it is possible to make the reaction surface area that reacts with the electrolyte larger than that of cathode active material having the same particle size and a solid structure, so it is possible to greatly reduce the cathode resistance.
Cathode active material is known to inherit the characteristics of the transition metal composite hydroxide particles of the precursor. In other words, in order to obtain the cathode active material described above, it is necessary to suitably control the particle size, the particle size distribution, and the specific surface area of the precursor transition metal composite hydroxide particles.
For example, JP 2012-246199 (A), JP 2013-147416 (A), and WO 2012/131881 disclose methods of producing transition metal composite hydroxide particles that become the precursor of cathode active material by a crystallization reaction that is clearly divided into two stages: a nucleation process that mainly performs nucleation, and a particle growth process that mainly performs particle growth. In these methods, the pH value of the reaction solution at a standard liquid temperature of 25° C. is controlled to be in the range 12.0 to 13.4 or 12.0 to 14.0 in the nucleation process, and in the range 10.5 to 12.0 in the particle growth process. Moreover, the reaction atmosphere is an oxidizing atmosphere in the nucleation process and at the beginning of the particle growth process, and at specified timing, is switched to a non-oxidizing atmosphere.
The transition metal composite hydroxide particles that are obtained by such methods have a small particle size and narrow particle size distribution, and has a low-density center section comprising fine primary particles, and a high-density outer shell section comprising plate-shaped or needle-shaped primary particles. Therefore, when such transition metal composite hydroxide particles are fired, the low-density center section contracts a large amount, and a hollow section is formed on the inside. As described above, the cathode active material inherits the particle characteristics of the composite hydroxide particles. More specifically, the cathode active material that is obtained by the technology that is disclosed in the above literature has an average particle size in the range of 2 μm to 8 μm, or 2 μm to 15 μm, an index [(d90−d10)/average particle size] that indicates the range of the particle size distribution of 0.60 or less, and the structure is a hollow structure. Therefore, in rechargeable batteries that use these cathode active materials, the capacity characteristics, output characteristics, and cycling characteristics are considered to be simultaneously improved.
However, the output characteristics of the rechargeable batteries that use these cathode materials cannot be said to be sufficiently improved. Particularly when considering usage as the power source of an electric automobile such as described above, it is necessary to further improve the output characteristics without impairing the capacity characteristics and cycling characteristics.